Short term peak shaving of natural gas

ABSTRACT

Apparatus and method for &#39;&#39;&#39;&#39;peak shaving&#39;&#39;&#39;&#39; storage of gas in a gas distribution system wherein the pressure varies from a maximum when demand is low to a minimum when demand is high, the method comprising cooling gas, in a gas storage chamber in open fluid communication with the system with a frozen cooling medium having a freezing point approximately midway between the minimum and maximum boiling points of the gas in the system. When the pressure in the system is high, gas in the chamber liquifies and is stored; when the pressure in the system is low gas in the chamber vaporizes and is available for use.

United States Patent 51 3,661,542 Collins May 9, 1972 I 541 SHORT TERM PEAK SHAVING 0 3,302,416 2/1954 Proctor et a1. ..62/54 X 3,302,416 2/1954 Proctor et a1. ..62/54 X NATURAL GAS Michael H. Collins, Frodsham, England Shell Oil Company, New York, N.Y.

Jan. 9, 1970 lnventor:

Assignee:

Filed:

Appl. No:

Foreign Application Priority Data Jan. 23, 1969 Great Britain ..3,955/69 US. Cl ..48/19l,62/45, 62/52, 62/54 Int. Cl ..F17d1/04 Field of Search ..62/50, 51, 52, 53, 54, 55, 62/45, 8-44, 174; 48/190, 191, 196

References Cited UNITED STATES PATENTS 10/1961 Vanderlee et a1 ..62/174 Primary Examiner-Meyer Perlin Assistant E.raminer-Ronald C. Capossela Attorney-J. H. McCarthy and T. E. Bieber [57] ABSTRACT Apparatus and method for peak shaving storage of gas in a gas distribution system wherein the pressure varies from a maximum when demand is low to a minimum when demand is high, the method comprising cooling gas, in a gas storage chamber in open fluid communication with the system with a frozen cooling medium having a freezing point approximately midway between the minimum and maximum boiling points of the gas in the system. When the pressure in the system is high, gas in the chamber liquifies and is stored; when the pressure in the system is low gas in the chamber vaporizes and is available for use.

17 Claims, 3 Drawing Figures CONSUMER PATENTEBMAY 9 I972 .SHORT TERM PEAK SHAVING OF NATURAL GAS BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to a system for the storage of gas, and particularly the temporary storage of natural gas.

2. Description of the Prior Art Consumption of energy producing resources fluctuates between wide limits and provision has to be made for storage of surplus fuel in time of low consumption and the supplementing of the fuel supply when consumption is high. Gas presents particular problems in that the main storageis bulky, as the consumer usually draws on a common supply and does not himself store the fuel. In order that the best possible use, economically, is made of the source and pipework involved in a gas distribution system it is usual to determine the demand to be met and then to design the gas producing system so that the initial output of gas is maintained at a constant rate, the pressure of gas in the pipeline varying with demand but not exceeding the safe working pressure of the piping.

In a natural gas distribution network it is usual practice to employ a large diameter line of high pressure pipe from the source (which may supply gas at pressures of the order of 35 atmospheres) to the main areas of consumption. If necessary, pumps may be provided at various points along the line to ensure that a suitably high pressure is maintained. From this trunk line, smaller lower pressure feeder lines are branched off to the various areas and these are further branched to supply individual consumers. At present trunk lines are usually maintained at a maximum pressure of about 30 atmospheres. Such pressures are experienced at periods of lowest consumption, while when consumption increases the pressure may drop to as low as 12 atmospheres. Provision is made for peak shaving of the supply when consumption is low, some of the gas in the line being removed, usually by liquefaction, and stored for regasification and, distribution in the network during periods of high consumption.

SUMMARY OF THE INVENTION We have now found that it is possible to provide a relatively simple storage method, particularly for short term storage. of natural gas, which is automatically responsive to, and iscontrolled by, consumer demand. The method makes use of the fact that, as is well known, the boiling point of a liquefiedgas is not constant but is dependent upon the pressure applied thereto, and consists essentially of-exposing the gas to a temperature such that at the high pressures which apply attimes of low consumption some of it is liquefied, but at the low pres.- sures which occur during periods of high demand the boiling point of the liquid is exceeded so that it vaporizes and passes into the system.

As mentioned above, it is usual to operate gastrunk lines so that the gas pressure is of the order of 30 atmospheres at periods of low demand and at this pressure the boiling pointof natural gas (which is essentially a mixture of lower hydrocarbons, e. g. methane, ethane and propane; the major constituent is methane, which has a normal boiling point of 116 K) is, about 175 K. By cooling such a gas to a temperature of'about 165 K, at a pressure of about 30 atmospheres it will be ata temperature below its boiling point and consequently will liquefy. Conversely when demand increases to the point at which the pressure in the line drops to such a degree that the boiling point of the liquefied gas is below 165 K it will boil and the gas so generated may be fed into the delivery line. The pressure may drop to as low as 12 atmospheres, at which pressure the boiling point of natural gas is about 155K.

Maintenance of a temperature between the maximum and minimum boiling points of the gas under the pressure conditions likely to be found in a supply system may be, for example, by the provision of suitably controlled refrigeration means, but we prefer to use as the cooling medium a substance having a phase transition temperature (preferably a solid/liquid phase transition temperature) between the before mentioned maximum and minimum boiling points. Preferably the phase transition temperature of the cooling medium (for convenience we shall hereinafter refer to this cooling medium as the cryogenic medium") will lie about midway between the maximum and minimum boiling points of the natural gas. By use of such a cryogenic medium a substantially reversible situation may be created whereby the heat gains and losses associated with changes in physical state at a constant temperature, the so-called latent heat changes, may be utilized so that under conditions of low consumption, when the pipeline pressure is high, gas from the pipeline which is in heat transfer contact with the cryogenic medium in one phase is liquefied, losing heat in the process tothe cryogenic medium, so that this also changes its phase; and when subsequently the pressure in the pipeline drops during a period of high consumption, a proportion of the stored liquefied gas vaporizes, withdrawing heat from the cryogenic medium which is thereby cooled and converted back to its original state.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a gas distribution system suitable for use according to theprocess of this invention.

FIG. 2 is a vertical cross section of one latent heat storage/exhange unit which may be used in practicing this invention.

FIG. 3 is a vertical cross section of an alternative latent heat storage/exchange unit which may be used in the practice of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the figures, in one embodiment of this invention a consumer demandresponsive device for storage of fuel gas, controlled by the pressure of gas in the distribution system, comprises a storage chamber for the liquefied gas, for example a storage tube l0v branching (in a downwards direction so that gravity collection of the liquefied gas occurs) off from and open to a main gasline, 1-1 which is part of a system of gas lines (notshown) delivering gas from a source 12 to a point of consumption. 13. The. storage tube 10 is preferably surrounded at least in part. by a container l4 which holds a cyrogenicmedium 20thatis at a temperature near its freezing point. The cryogenic medium 20, which may, for example, be a hydrocarbon, preferably hasa freezing point approximately midway between the boiling points of the gas containedrin the system at the pressure extremes occurring at that point in thesystemat which=the storagetube 10 is located. So that as little as possible ofthe heat being exchanged between the gasand thecryogenic medium may belost, it is desirable that the outer walls l5 of the container for the cryogenic mediumshould. be insulating. In some-cases thispart of the container 14 may be manufactured from an insulating material e.g. foamed polyurethane, or it may. be a shell, e.g. of metal, on to which'insulant is attached or coated.

The gas inadistribution system will normally be ata temperature closely approximating ambient and the difference between this temperature and that of the cryogenic medium 20 will be so great thatthe quantity of heat required by the cryogenic medium 20to complete its phase change will be reached while just cooling down the bulk of the gas and before much of it is condensed. vOn subsequent revaporization of the liquefied gas little of-the cryogenic medium 20 will be able to return to its original phase, and thus the efficiency of the liquefaction-vaporization cycle and the number of cycles that can be successively carried out without the requirement to return the cryogenic medium to its original phase and temperature will be reduced. To reduce the difference in temperature between gas andthe cryogenic medium. 20 when they come into contact we prefer to provideforprecooling of the gas using a precooler means 17 which may be a pebble-bed heat exchanger or some other heat storage/exchange system between the main gas line 11 and that regionv of the storage tube 10 surrounded by the'cryogenic medium container 14.

The container 14 houses a second cooling means which is preferably a latent heat storage/exchange system comprising a device whereby the cooled'gas passing down the storage tube from the precooler 17 (in the direction of arrow 19) is brought into heat exchange contact with a suitable cryogenic medium 20 at a temperature which is below its phase transition temperature, this being approximately midway between the boiling points of the gas at the maximum and minimum pressures that occur at that point in the system. In the embodiment of FIG. 2 the walls of the storage tube 10 act as the inner wall of the container 14 and, thus, provide a surface for heat exchange between the cryogenic medium 20 and gas inside the storage tube 10.

FIG. 3 shows an alternative embodiment of the latent heat storage/exchange unit in which a cryogenic medium 20 is contained within small substantially spherical containers 22 which may, for example, be of gelatine. In this embodiment the walls of the spherical containers 22 provide a surface for heat exchange between the cryogenic medium 20 and gas which has passed down the storage tube 10 from the precooler l7 and entered a heat exchange chamber 23 in a container 24 which holds the cryogenic medium filled spherical containers 22. In both this embodiment and that of FIG. 2 when some of the liquefied gas revaporizes, using heat drawn from the cryogenic medium 20 and causing the latter to revert to its original phase, it passes back (in the direction of arrow 21) through the precooler 17 where it is warmed, preferably to ambient temperature again, while recooling the precooler 17.

As suggested earlier in this specification, some or all of the equipment used in the gas storage system may be insulated as considered necessary. Because of the relatively small size and compact nature of the units of the present invention, they may be made transportable and therefore sufficiently mobile to be of use to meet temporary requirements or possibly as a means of facilitating tramsportation of gases by ship in the liquid form and revaporization upon delivery.

For example it is envisaged that a storage unit in accordance with the invention could be used to liquefy gas in the vicinity of a supply source and the liquefied gas could then be transported to a suitable consumer supply station where, by reduction of pressure, evaporation of the liquefied gas would result in cooling of the cryogenic medium, which could be used subsequently to liquefy a further supply of gas at high pressure.

The design of the area in which liquefaction and storage of the gas takes place will have an effect on the overall efficiency of the system, since some shapes will assist the rapid transfer of heat into and out of the cryogenic medium by ofiering a large surface and enabling the cryogenic medium to remain in contact with the surface during and after its phase change. That is, the usual rules regarding heat exchange techniques are adhered to. Thus the material from which the surface between the gas and the cryogenic medium is manufactured should have as large a heat conductivity as possible in order to facilitate the heat transfer through it. Examples of heat exchange units according to the above, preferably having the surfaces between gas and cryogenic medium fabricated from a metal such as copper, include those where gas is contained within a space surrounded almost completely by the cryogenic medium .(as in the embodiment of FIG. 2) or the reverse wherein the gas passes over and around the surface of a container or containers wherein the cryogenic medium is stored (as in the embodiment of FIG. 3). Provision may also be made to allow condensate, either undesirable condensates or possibly liquefied gas for separate storage, to be drawn off as through valves 25 and 26.

The short-term peak shaving systems of the present invention will, because of the very low temperature at which they will be operating and the fact that perfect insulation will not be possible to achieve, together with the fact that the phase change cycles involved will not be completely reversible, tend to use up the cold" from the cryogenic medium 20 and therefore require the latter to be cooled from time to time. This will most conveniently. be efie'cted by the addition to the unit of liquefied. gas from some outside source as through valves 25 and 26 in FIGS. 2 and 3. it is thought that the quantities of liquefied gas needed for this purpose will be quite small, amounting, perhaps, when the system is applied to a natural gas nationwide distribution system, to about 1,250 gallons for every 78,000 gallons produced by the unit. This is about onesixtieth of the amount required when peak shaving is carried out by revaporization of liquefied gas into the system.

Although the choice of cryogenic medium 20 is dependent upon consideration of the phase change to be used, temperature of phase change required, which is dependent upon the type of gas encountered and the pressures of it, and the resources and finance available, we have found that for a unit to be used for the short-term peak shaving of natural gas in a distribution network operating at a maximum line pressure in the region of 30 atmospheres, iso-octane, which has a melting point in the required pressure range about K, is particularly suitable. It should be noted though that by the use of suitable pressure equipment for its storage natural gas itself could be utilized as the cryogenic medium 20. Of course, where different ranges of boiling point, for example where the gas to be stored is not natural gas, are encountered, selection of the cryogenic medium 20 will be made accordingly in the light of the preceding comments.

lt will be appreciated that although a major emphasis has been put on the application of the invention to natural gas distribution systems, and in particular to large scale distribution of this gas, the invention may equally well be utilized with any gas distribution system carrying a gas which will condense/vaporize at different temperatures dependent upon the pressure exerted upon it. Selection of a cryogenic medium having a phase transition temperature between the boiling points will present little difficulty to the skilled man.

i claim as my invention:

1. In a gas distribution system containing a gas wherein the pressure within the system varies as from a maximum at times of low demand to a minimum at times of peak demand causing a simultaneous change in the boiling point of the gas in the system from a maximum boiling point to a minimum boiling point, a method for the storage of gas comprising the steps of:

providing a gas storage chamber in open fluid communication with the gas distribution system whereby the pressure. in said storage chamber is maintained substantially equal to the pressure in said system;

maintaining the gas storage chamber full of gas from said system; cooling said gas storage chamber and its contents to a temperature approximately midway between the minimum and maximum boiling points of the gas in the system; and

maintaining said temperature in said gas storage chamber substantially constant at said temperature approximately midway between the minimum and maximum boiling points of the gas in the system as the pressure within said system varies whereby gas condenses in said storage chamber at times of low demand and vaporizes from said storage chamber at times of peak demand.

2. The method of claim 1 including the step of precooling at least some of the gas prior to its entering the storage chamber.

3. The method of claim 1 wherein said cooling of the storage chamber and its contents is achieved by transfer of heat to a cryogenic medium having a phase transition temperature approximately midway between the maximum and minimum boiling points of the gas in the system.

4. The method of claim 3 wherein when the pressure in said gas storage chamber is increased, at least some of the gas in said gas storage chamber is liquified and at least some of said cryogenic medium undergoes a phase transition in which latent heat is consumed and when the pressure in said gas storage chamber is decreased at least some of the gas in said gas storage chamber is vaporized and at least some of said gas cryogenic medium undergoes a phase transition in which latent heat is yielded.

5. The method of claim 4 wherein said gas is natural gas.

6. The method of claim 5 wherein said cryogenic medium is iso-octane.

7. The method of claim 1 including the step of periodically removing liquified gas from said gas storage chamber for separate storage.

8. The method of claim 4 including the step of periodically cooling said cryogenic medium by adding a volume of liquified gas to said gas storage chamber.

9. An apparatus for the storage of a gas in a gas distribution system wherein the pressure varies through a range of pressures from a maximum pressure when demand is low to a minimum pressure when demand is high, the apparatus comprising:

gas storage container means forming a chamber in open fluid communication with said gas distribution system whereby the pressure in said gas storage container means is maintained substantially equal to the pressure in said gas distribution system;

heat storage/exchange means operatively associated with said gas storage container means for liquifying gas in the gas storage container means when the pressure in said gas storage container means is substantially in the upper half of the pressure range through which pressure in the gas distribution system varies and for vaporizing gas in the gas storage container means when the pressure in said gas storage container means is substantially in the lower half of said pressure range;

said heat storage/exchange means including a cryogenic medium having a phase transition temperature midway between the boiling point of said gas when the pressure in the system is at said maximum pressure and the boiling point of said gas when the pressure in the system is at said minimum pressure.

10. The apparatus of claim 9 including conduit means including valve means opening into said gas storage means and disposed so that liquified gas may be periodically withdrawn from or added to said gas storage means through said conduit means.

11. The apparatus of claim 9 including precooler heat exchange means disposed to precool gas moving into said gas storage container means.

12. The apparatus of claim 9 wherein the heat storage/exchange means includes a cryogenic medium container substantially surrounding said gas storage container means, the cryogenic medium container having heat insulating exterior walls in spaced relationship with heat conductive interior walls to form a closed space between said exterior and interior walls to contain said cryogenic medium.

13. The apparatus of claim 9 wherein said heat conductive interior walls form at least a part of said gas storage container means.

14. The apparatus of claim 9 wherein said heat storage/exchange means includes a plurality of cryogenic medium containers having heat conductive walls disposed within said gas storage container means so that each of said cryogenic medium containers may be substantially surrounded by the gas.

15. The apparatus of claim 11 wherein said precooler heat exchange means is a pebble-bed heat exchanger.

16. The apparatus of claim 9 wherein said cryogenic medium is a hydrocarbon.

17. The apparatus of claim 16 wherein said cryogenic medium is iso-octane. 

2. The method of claim 1 including the step of precooling at least some of the gas prior to its entering the storage chamber.
 3. The method of claim 1 wherein said cooling of the storage chamber and its contents is achieved by transfer of heat to a cryogenic medium having a phase transition temperature approximately midway between the maximum and minimum boiling points of the gas in the system.
 4. The method of claim 3 wherein when the pressure in said gas storage chamber is increased, at least some of the gas in said gas storage chamber is liquified and at least some of said cryogenic medium undergoes a phase transition in which latent heat is consumed and when the pressure in said gas storage chamber is decreased at least some of the gas in said gas storage chamber is vaporized and at least some of said gas cryogenic medium undergoes a phase transition in which latent heat is yielded.
 5. The method of claim 4 wherein said gas is natural gas.
 6. The method of claim 5 wherein said cryogenic medium is iso-octane.
 7. The method of claim 1 including the step of periodically removing liquified gas from said gas storage chamber for separate storage.
 8. The method of claim 4 including the step of periodically cooling said cryogenic medium by adding a volume of liquified gas to said gas storage chamber.
 9. An apparatus for the storage of a gas in a gas distribution system wherein the pressure varies through a range of pressures from a maximum pressure when demand is low to a minimum pressure when demand is high, the apparatus comprising: gas storage container means forming a chamber in open fluid communication with said gas distribution system whereby the pressure in said gas storage container means is maintained substantially equal to the pressure in said gas distribution system; heat storage/exchange means operatively associated with said gas storage container means for liquifying gas in the gas storage container means when the pressure in said gas storage container means is substantially in the upper half of the pressure range through which pressure in the gas distribution system varies and for vaporizing gas in the gas storage container means when the pressure in said gas storage container means is substantially in the lower half of said pressure range; said heat storage/exchange means including a cryogenic medium having a phase transition temperature midway between the boiling point of said gas when the pressure in the system is at said maximum pressure and the boiling point of said gas when the pressure in the system is at said minimum pressure.
 10. The apparatus of claim 9 including conduit means including valve means opening into said gas storage means and disposed so that liquified gas may be periodically withdrawn from or added to said gas storage means through said conduit means.
 11. The apparatus of claim 9 including precooler heat exchange means disposed to precool gas moving into said gas storage container means.
 12. The apparatus of claim 9 wherein the heat storage/exchangE means includes a cryogenic medium container substantially surrounding said gas storage container means, the cryogenic medium container having heat insulating exterior walls in spaced relationship with heat conductive interior walls to form a closed space between said exterior and interior walls to contain said cryogenic medium.
 13. The apparatus of claim 9 wherein said heat conductive interior walls form at least a part of said gas storage container means.
 14. The apparatus of claim 9 wherein said heat storage/exchange means includes a plurality of cryogenic medium containers having heat conductive walls disposed within said gas storage container means so that each of said cryogenic medium containers may be substantially surrounded by the gas.
 15. The apparatus of claim 11 wherein said precooler heat exchange means is a pebble-bed heat exchanger.
 16. The apparatus of claim 9 wherein said cryogenic medium is a hydrocarbon.
 17. The apparatus of claim 16 wherein said cryogenic medium is iso-octane. 